Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. By providing a desired engine dilution, such systems reduce engine knock, in-cylinder heat losses, throttling losses, as well as NOx emissions. As a result, fuel economy is improved, especially at higher levels of engine boost. However, cooled EGR is limited by the ability of the combustion system to maintain acceptable stability and combustion rates while diluted with EGR.
Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR to other engine cylinders. Therein, exhaust from only the dedicated cylinder group is recirculated to remaining engines. As such, this allows a substantially fixed amount (e.g., percent) of EGR to be provided to engine cylinders at most operating conditions. By adjusting the fueling of the dedicated EGR cylinder group, the EGR composition can be varied. In particular, the dedicated cylinder can be operated richer than stoichiometry to produce more combustible species, such as hydrogen and carbon monoxide, which when rerouted to the engine intake can increase EGR tolerance. As such, this further improves the fuel economy benefit coming from EGR by decreasing combustion instability and burn durations while increasing allowable EGR rates.
One example of a boosted engine system having dedicated EGR cylinder capabilities is shown by Gingrich et al. in US 20120204844. Therein, exhaust gas from a dedicated EGR cylinder is mixed with boosted air from a compressor at a location upstream of a charge air cooler and upstream of an intake throttle so that cooled EGR can be delivered to the engine.
However, the inventors herein have recognized potential issues with such dedicated EGR cylinder configurations. In particular, EGR may be at the wrong level during transients. As an example, if there is a sudden increase in torque demand and a change in the throttle position to a more open position with a consequent drop in EGR demand, due to the specific location of EGR delivery, there may be manifold filling delays that result in more EGR residuals remaining in the manifold than desired. The long delay in purging the EGR residuals may lead to a drop in boosted engine performance. As another example, if there is a sudden decrease in torque demand and a change in throttle position to a more closed position with a consequent rise in EGR demand, due to the location of EGR delivery and resulting manifold filling delays, there may be fewer EGR residuals than desired. The long delay in filling the manifold with EGR may lead to a drop in engine performance. In both cases, combustion stability issues may also arise. While diverter valves may be used for diverting some or all of the exhaust from the dedicated EGR cylinder to an exhaust location during conditions when EGR is not required, the use of diverter valves may be cost prohibitive in addition to suffering from durability issues.
The inventors have recognized these and issues and have at least partly addressed them by a method for an engine that allows exhaust from the dedicated EGR cylinder to be routed to a plurality of locations based on engine operating conditions. The method comprises: selectively opening a plurality of exhaust valves of a dedicated EGR cylinder group to recirculate exhaust gas to remaining engine cylinders at each of a pre-compressor and a post-compressor location. In this way, EGR delivery locations and rates can be easily varied as engine operating conditions change.
In one example, an engine system may be configured with a single dedicated EGR (DEGR) cylinder for providing external EGR to all engine cylinders. Further, the engine may include a charge air cooler (CAC) integrated into the intake manifold allowing for compaction of the boosted engine system. The dedicated EGR cylinder may include a plurality of exhaust valves, for example three exhaust valves, and a single intake valve. A first exhaust valve may direct exhaust from the DEGR cylinder to the engine intake, downstream of the charge air cooler and downstream of an intake throttle, thereby allowing hot EGR to be delivered at a more downstream location of the intake manifold. A second exhaust valve may direct exhaust from the DEGR cylinder to the engine intake, upstream of the intake compressor, thereby allowing cooled EGR to be recirculated at a more upstream location of the intake manifold. Since the engine includes an integrated CAC, both the hot and cooled EGR is delivered into a smaller manifold volume, reducing manifold filling delays and expediting EGR filling even if a throttle position suddenly changes. A third exhaust valve may divert hot exhaust from the DEGR cylinder to the exhaust manifold, at a location upstream of an exhaust catalyst while bypassing the remaining engine cylinders. Thus, the third exhaust valve allows no EGR to be delivered to the engine. The plurality of exhaust valves may be operated with variable valve timing, such as through the use of a cam profile switching (CPS) mechanism, so that one or more of the exhaust valves are selectively activated at a given time. For example, the CPS mechanism may be used so that the dedicated EGR cylinder is operated in one of a plurality of modes based on engine operating conditions, the mode determining when and for how long each exhaust valve is opened during an exhaust stroke in the dedicated EGR cylinder group. As an example, during low load and/or low boost conditions, the dedicated EGR cylinder may be operated in a first mode where only the first exhaust valve is opened during the exhaust stroke so that hot EGR is delivered to the engine intake at a post-compressor location. Then, during high load and/or high boost conditions, the dedicated EGR cylinder may be operated in a second mode where only the second exhaust valve is opened during the exhaust stroke and cooled EGR is delivered to the engine intake at a pre-compressor location. In comparison, during engine cold-start or catalyst warm-up conditions, or when no engine dilution is required, the dedicated EGR cylinder may be operated in a third mode with only the third exhaust valve opened during the exhaust stroke to deliver hot exhaust to the exhaust catalyst while bypassing the engine cylinders. In still further examples, the timing of each of the exhaust valves may be adjusted so that they operate with varying degrees of overlap (e.g., only partial overlap).
In this way, a location of EGR delivery from a dedicated EGR cylinder group can be varied as operating conditions change in a boosted engine system. By allowing EGR to be selectively delivered to a pre-compressor and/or a post-compressor location based on engine load and boost conditions, combustion stability issues and EGR errors during transient changes in throttle position can be reduced. By introducing EGR to a pre-compressor location at higher engine loads and higher EGR rates, EGR cylinder-to-cylinder balance is improved while also providing additional EGR cooling through a charge air cooler. Furthermore, compressor surge risk is reduced. By varying the location of EGR delivery based on engine operating conditions, a discrepancy between the location where EGR is required at those conditions and the location where EGR is introduced is lowered, thereby reducing the amount of EGR delivery errors generated, as well as decreasing manifold filling times. As such, this reduces the likelihood of EGR being at the wrong level. In addition, the approach allows for simultaneous sourcing of cooled and uncooled EGR at a reduced rate along with some exhaust flow to the catalyst for light off maintenance. By adjusting the opening of a plurality of exhaust valves of a dedicated EGR cylinder group, the blow down portion of an exhaust stroke can be leveraged for improved EGR driving capability. Further, the exhaust can be routed to a pre-turbine location for improved turbocharger performance. Likewise, the exhaust opening may be adjusted to leverage the scavenging portion of the exhaust stroke for a greater concentration of unburned and partially burned hydrocarbons for improved EGR tolerance in the engine. By reducing EGR errors during transients, boosted engine performance, even with high engine dilution, is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.